Ultrasonic devices may be used in a wide variety of applications, such as acoustic pressure sensors for ultrasonic field characterization. A hydrophone is a type of ultrasonic device that has been employed as an acoustic pressure sensor for calibrating an ultrasonic transducer used in medical diagnosis and therapy. Calibration of the ultrasonic transducer can be achieved by directing waves from the transducer to the hydrophone. The hydrophone is operated to provide a quantitative assessment of the characteristics of the ultrasonic field that is created by the transducer in a liquid, such as water.
Performance properties such as sensitivity, frequency response, acoustic transparency and immunity to rf interference must be considered in the design of a hydrophone for ultrasonic field characterization. One type of hydrophone design is a needle-like device described in U.S. Pat. No. 4,789,971 to Powers. et al. Despite the small size of the needle-like hydrophone, this type unavoidably changes the ultrasonic field that is to be characterized. Perturbations of the field are generated as a result of the geometry of the hydrophone and the substantial difference in acoustic impedance between the hydrophone and the liquid in which the hydrophone is immersed.
A membrane hydrophone is a type of device that is generally more acoustically transparent than the needle-like devices. Membrane hydrophones are described in U.S. Pat. Nos. 4,433,400 to DeReggi et al. and 4,653,036 to Harris et al. Such hydrophones typically include a thin polyvinylidene fluoride (PVDF) film that is held taut by a rigid hoop. PVDF membranes are employed because the acoustic impedance of PVDF is relatively close to that of water. Impedance matching reduces the reflections generated by the hydrophone. Moreover, the diameter of the hoop is typically several times as large as the diameter of the acoustic beams that are to be encountered, so that the hoop is less likely to generate perturbations.
The manufacture of a membrane hydrophone is described in the above-identified patent to DeReggi et al. The PVDF membrane may be a single sheet or may be a bilaminate member. The membrane is clamped between inner and outer hoop rings that may be made of brass. The center of the membrane is poled to provide an active sensing area. The active area is strongly piezoelectric and typically is smaller than the wavelength of the highest frequency to be encountered. For example, the active area may have a diameter of approximately 0.5 mm. Electrodes are formed on the opposed sides of the active area and leads extend from the electrodes for the conduction of an electrical signal from the active area. The electrodes and leads may be deposited on the PVDF by vacuum evaporation through a metallic mask. The electrodes and leads may also be formed photolithographically.
DeReggi et al. also describes using a silicon rubber to fix a preamplifier to the PVDF membrane. The preamplifier is used to achieve impedance matching for electrical connection to a coaxial transmission line that is connected to the hoop. A typical membrane additionally has a metallized ground plane coating on at least one exterior surface in order to achieve rf interference shielding.
It is an object of the present invention to provide a membrane acoustic device and a method of fabricating such a device utilizing techniques which integrate the various components necessary to achieve a high sensitivity over a wide range of frequencies.